1. Field of the Invention
The present invention relates to a fixing device for fixing a toner image transferred onto a sheet and an image forming apparatus employing such a fixing device.
2. Description of the Related Art
There have been conventionally known image forming apparatuses constructed such that an electrostatic latent image on the outer circumferential surface of a photoconductive drum is developed with toner to form a toner image, and an output image is obtained by transferring this toner image to a sheet. There have been also known fixing devices installed in such image forming apparatuses to fix a toner image transferred onto a sheet.
A conventional fixing device is provided with a fixing (heat) roller for heating a sheet and a pressure (press) roller for pressing the sheet against the fixing roller while defining a nip area in cooperation with the fixing roller. In the fixing device constructed in this way, the sheet may, in some cases, wind around the fixing roller by being conveyed from the nip area between the two rollers while being adhered to the fixing roller. In order to prevent the winding of the sheet, the fixing device is generally provided with a separating claw for separating the sheet from the fixing roller. This separating claw is arranged in the vicinity of a downstream end of the nip area and also has a function of guiding the sheet separated from the fixing roller to a downstream side of a conveyance path.
However, since the separating claw guides the sheet while separating it as described above, there are cases where an image formed on the sheet is scratched by the separating claw. This is because the separating claw constantly in contact with the fixing roller comes to have a high temperature and the sheet comes into contact with the separating claw having a high temperature for a specified period, thereby melting (remelting) the toner on the sheet.
As means for solving this problem is known an art of guiding the sheet separated from the fixing roller by the separating claw onto a guiding member arranged at a relatively close position from a separating position (tip of the separating claw) and guiding the sheet to the downstream side of the conveyance path by means of the guiding member. Since this guiding member is provided separately from the separating claw and kept at a lower temperature than the separating claw, the melting of the toner on the sheet is suppressed upon being held in contact with the guiding member for a specified period.
However, in the above construction, the guiding member needs to be arranged in the vicinity of the downstream end of the nip area between the two rollers, the sheet may be, in some cases, conveyed from the nip area while being strongly pressed against the guiding member due to the elasticity thereof. This causes a problem of creating image abrasion (guiding trace) on the outer surface of the image on the sheet.
As a countermeasure against such an image abrasion, an art of guiding a sheet separated from a fixing roller to a downstream side of a conveyance path by means of a rotatable roller or the like has been proposed, for example, in Japanese Unexamined Patent Publication No. 2004-61854. This patent publication discloses a fixing device in which a rotary member is attached to a separating-claw protecting member that pivots together with a separating claw. This separating-claw protecting member turns together with the separating claw in a direction away from the fixing roller at the time of a sheet jam, thereby preventing the separating claw from being excessively pressed against the fixing roller to damage the separating claw and the fixing roller. Further, the sheet separated from the fixing roller by the separating claw is guided to the rotary member without coming into contact with the separating-claw protecting member, thereby preventing an occurrence of an image abrasion on the sheet after an image fixing operation.
However, this patent publication discloses no member for actively guiding the sheet separated by the separating claw toward the rotary member. Accordingly, in the case where a separating claw 70 and a roller member (rotary member) 80 are arranged, for example, to have such a positional relationship as to largely change a conveyance direction of a separated sheet P as shown in FIG. 24, it is difficult to smoothly bring the sheet P into contact with the outer circumferential surface of the roller member 80 and a large load acts on the sheet P. This causes an occurrence of a bridge phenomenon called “pull-back” of the sheet P resulting from the elasticity of the sheet P, thereby causing a problem of bringing a bridged part of the sheet P into contact with the separating claw 70. Since this causes the separating claw to scratch the sheet, there is a problem of reducing image quality.